Shape evolution mechanism and characterization of two-dimensional MoS₂ and NbS₂

Two dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have attracted much attention due to their unique physical and chemical properties. Increased interest in the research of 2D layered materials began in 2004 when graphene, a monolayer of graphite, was discovered. TMDs are a class of layered materials which consist of over 40 members ranging from semiconductors to insulator to metals. There are three polytypes crystal structure of TMDs (1T, 2H, and 3R), which represent trigonal, hexagonal and rhombohedral, respectively. TMDs can be synthesized by one of two different approaches, top-down and bottom-up. Top-down includes mechanical and chemical exfoliation, while bottom-up includes physical and chemical vapor deposition. Synthesizing these atomically thin materials enables a new set of properties due to the quantum confinement effects. MoS2 and NbS2 are two significant members of the TMD family and are the main topic of work in this thesis. These two materials were successfully synthesized with chemical vapor deposition (CVD) by controlling their growth parameters, each of which has a great influence on nucleation and the growth mechanism. Their shape evolution mechanism provides important insight into their nucleation and growth mechanism. The MoS2 part of this work was dedicated to understand this mechanism and how to apply it to other TMDs. To do that, MoS2 was synthesized on different substrates to explore the optimum surface chemistry between the substrate atoms and reactants. On the other hand, Niobium disulfide (NbS2) has received less attention compared to other TMDS due to its unexplored properties. Therefore, analyzing growth parameters and the potential applications of NbS2 was also another aim of this thesis. In the NbS2 part of this thesis, two different approaches were pursued growing NbS2 flakes. The first approach used sulfur and Nb2O5 powders as the precursors. The second approach used an alkali promoter, Nb2O5, and sulfur. The alkali-assisted approach yielded high-quality growth of few-layered NbS2 with thicknesses ranging from 2 – 10 nm and lateral dimensions in the hundreds of micrometers. Our Raman and transmission electron microscopy analyses suggested that the as-grown material is a mixture of 2H and 3R phases. It was found that there was a substantial amount of incorporated potassium ions from the growth process in the NbS2 samples, which leads to doping and metallic behavior. In the absence of alkali growth promoters, the lateral dimensions of the crystals are smaller, but they displayed metallic behavior. The electrical and HER properties of NbS2 exhibited a promising result for future studies. The realization of thin and metallic TMDs is crucial for studying potential integration into electronics and catalysis.Ph.D.Includes bibliographical referencesby Ibrahim H. Bozkur